/**
 * 斐波拉契数列
 */
pub fn run (n: u64) -> u64 {
    let mut a = 0;
    let mut b = 1;
    let mut result = 0;
    if n < 2 {
        return n;
    }
    for _i in 2..n + 1 {
        result = a + b;
        a = b;
        b = result;
    }
    result
}